The person hiding in the mirror was Illuso from the Hitman Team. Fugo ends up being dragged into the mirror by Illuso's Stand, Mirrorman. Fugo then summons his own Stand, Purple Smoke. Despite Giorno wanting to find Fugo, Abbacchio decides to go look for the key, since that was their mission. However, once Abbacchio finds the key, he also gets dragged into the mirror world...
Second is slitting Giorno's throat first, 
then safely retrieving the key.
والثّاني هو شقّ حنجرة جورنو أوّلاً
ومن ثمّ استعادة المفتاح
Oder aber ich schneide ihm erst den Hals durch
und hole mir den Schlüssel ganz gemütlich.
La segunda es degollar a Giorno
y luego quedarme con la llave.
La segunda es degollar a Giorno
y luego quedarme con la llave.
Le deuxième,
Oppure posso prima tagliargli la gola,
e poi recuperare la chiave in sicurezza.
Ou posso cortar a garganta dele,
e pegar a chave com tranquilidade depois.
А второй: сначала перерезать горло Джорно,
а уже потом спокойно прикарманить ключ.